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ANTHRACENE

is then stirred until this layer melts and the temperature noted, this indi-
cating approximately the melting point. The fused mass is then stirred
with the same thermometer until naphthalene crystals again begin to form
and the column of the thermometer remains stationary for some time. This
is the solidifying point, which, with pure naphthalene, corresponds with
the melting point.

2.  Presence of Oily Products.—A packet of 50 grams of the naphtha-
lene wrapped in several thicknesses of filter-paper is subjected in a press
to a pressure of about 150 atmos. for 10 minutes, the paper being then
examined to see if it is stained by the oil absorbed.

3.  Behaviour towards Petroleum Ether.—2 grams of the naphtha-
lene are treated in a test-tube with light petroleum to see if a clear, colourless
solution is obtained.

4.  Behaviour towards  Sulphuric  Acid.—4  grams  are heated in
a test-tube in a water-bath with 4 grams of cone, sulphuric acid until a
clear solution is obtained, the colour being observed.    The sulphuric acid
solution is poured into about 40 c.c. of water to ascertain if the whole remains
clear and colourless.

5.  Stability towards Light.—2   grams of   the naphthalene   on   a
clock-glass are left for 1-2 hours in a desiccator over cone, nitric acid (not
fuming), the naphthalene being then examined to see if it has remained
unaltered or if it is coloured.

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* *

Pure commercial naphthalene is white or slightly yellow and melts at 79-6-
79-8°; it should not mark paper (test 2) and should volatilise completely if
heated on a water-bath; test 3 should yield a clear solution, at most pink or
reddish, which should remain clear on dilution ; it should dissolve completely
in petroleum ether (test 4) and should remain unchanged when subjected to
test 5 (slightly impure naphthalene becomes pale pink).

ANTHRACENE

Commercial anthracene is always impure, containing principally naph-
thalene, methylanthracene, carbazole, paraffin wax, phenanthrene, etc. The
technical examination of crude anthracene is limited to the determination
of the anthracene content and to tests for the presence of impurities which
are harmful in the coal-tar colour industry. "

1. Determination of the Anthracene.—Luck's method, based on
the oxidation of anthracene to anthraquinone by means of chromic acid,
is usually employed. In a flask with a capacity of about half a litre, fitted
with a tapped funnel and a reflux condenser, a boiling solution of I gram
of the anthracene in 45 c.c. of glacial acetic acid is treated with a solution
of 15 grams of crystallised chromic acid in 10 c.c. of glacial acetic acid and
10 c.c. of water, this liquid being added in small quantities so that the whole
addition requires about 2 hours. The solution is boiled for two hours
longer, then left to itself for 12 hours, next mixed with 400 c.c. of cold water
and left at rest for 3 hours. The precipitated anthraquinone is collected
on a filter and washed first with cold water, then with faintly alkaline(including naph-